tests: Fix incorrect check name 'prop_name_chars'

'prop_name_chars' is not a valid check name, but the test was passing due
to a bug in dtc-checkfails.sh. Fix it to be the correct name,
'property_name_chars'.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>